XAT 2021 Question 39


Read the following passage and answer the three questions that follow.

Considering the multitude of situations in which we humans use numerical information, life without numbers is inconceivable. But what was the benefit of numerical competence for our ancestors, before they became Homo sapiens? Why would animals crunch numbers in the first place? It turns out that processing numbers offers a significant benefit for survival, which is why this behavioural trait is present in many animal populations.

Several studies examining animals in their ecological environments suggest that representing number enhances an animal’s ability to exploit food sources, hunt prey, avoid predation, navigate in its habitat, and persist in social interactions. Before numerically competent animals evolved on the planet, single-celled microscopic bacteria — the oldest living organisms on earth — already exploited quantitative information. The way bacteria make a living is through their consumption of nutrients from their environment. Mostly, they grow and divide themselves to multiply. However, in recent years, microbiologists have discovered they also have a social life and are able to sense the presence or absence of other bacteria; in other words, they can sense the number of bacteria. Take, for example, the marine bacterium Vibrio fischeri. It has a special property that allows it to produce light through a process called bioluminescence, similar to how fireflies give off light. If these bacteria are in dilute water solutions (where they are alone), they make no light. But when they grow to a certain cell number of bacteria, all of them produce light simultaneously. Therefore, Vibrio fischeri can distinguish when they are alone and when they are together.

Somehow they have to communicate cell number, and it turns out they do this using a chemical language. They secrete communication molecules, and the concentration of these molecules in the water increases in proportion to the cell number. And when this molecule hits a certain amount, called a quorum, it tells the other bacteria how many neighbours there are, and all bacteria glow. This behaviour is called “quorum sensing”: The bacteria vote with signalling molecules, the vote gets counted, and if a certain threshold (the quorum) is reached, every bacterium responds. This behavior is not just an anomaly of Vibrio fischeri; all bacteria use this sort of quorum sensing to communicate their cell number in an indirect way via signalling molecules.

Question 39

Which of the following statements CANNOT be inferred from the passage?


It is given that the ancestors of Homo sapiens interacted using numbers but we cannot infer that the ancestors of Homo sapiens interacted solely using numbers. In the second paragraph it is mentioned that they also used chemical language to communicate cell number.

The answer is option D.

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